The present invention relates to a conversion coating solution for treating metallic surfaces such as steel, galvanized steel sheets, tin-plated steel sheets and the like, particularly exposed steel surfaces treated by subjecting tin-plated steel sheets to drawing and ironing (hereinafter referred to as "DI processing"), for example, metallic surfaces such as tin-plated DI cans.
The surface treatment of metallic surfaces, particularly tin-plated DI cans, has heretofore used a solution for forming a passive chromate coating or a conversion coating solution containing an alkali phosphate as a main component. The former solution is used to form a passive chromate coating usually by rinsing the tin-plated DI can surfaces with a weak alkaline degreasing agent and then treating them with a treating solution containing chromic acid or a salt thereof. The coating or film can provide an anticorrosive property and can contribute to an increase in durability of the coating as an anticorrosive ground coat.
Chromium compounds contained in the treating solutions, however, present problems with respect to the environment when they are contained in waste water from a plant. They are also undesirable for use in processing of containers for food such as cans for food and drink because of their high toxicity.
The latter conversion coating solution containing an alkali phosphate as the main component contains acid alkali phosphates such as, sodium phosphate, potassium phosphate, ammonium phosphate or the like, and further contains an oxidizing agent such as NaClO.sub.3, NaBrO.sub.3, NaNO.sub.2, hydroxylamine salt or the like, or an additive of a halogen compound such as a fluoride as an agent for accelerating the formation of a film or coating.
Coatings or films obtainable by treating tin-plated DI can surfaces with the conversion coating solution as stated hereinabove can provide good adhesion of paints and a favorable gloss on the coated surfaces, but present the drawback of poor anti-corrosive performance because of the small amount of coating formed. In particular, tin-plated DI cans are exposed to the workshop atmosphere during the steps of rinsing with water, drying, over coating and printing after the conversion coating treatment step, so that rust is formed during these steps because coatings or films obtainable from conventional alkali phosphate coating solutions provide poor anti-corrosiveness; accordingly, the conventional technique has the defect of losing, after coating, the substantial effect produced by the conversion treatment.
Prior art solutions for treating metallic surfaces which contain tin ions are disclosed, for example, in Japanese Patent Publication No. 6,848/1971 (counterpart of U.K. Patent Specification No. 1,247,082), W. German Patent Publication (DE-AS) No. 1,199,100, and Japanese Laid-Open Patent Application No. 62,179/1980 (counterpart of U.S. Pat. No. 4,220,486).
Japanese Patent Publication No. 6,848/1971 involves conversion coating solutions for forming oxalate films on stainless steel and a process for treating the same. In this technique, the effect of the Sn.sup.++ action resides in the acceleration of etching on the stainless steel and forming an oxalate conversion coating thereon. When this treating solution is applied to tin-plated DI cans or tin-plated steel sheets, however, an oxalate coating having no gloss is formed with the treated surfaces, and no anticorrosive tin phosphate coating is formed thereon.
W. German Patent Publication No. 1,199,100 discloses conversion coating solutions for treating zirconium and its alloys and a process for forming coatings or films thereon. When these conversion coating solutions are applied to tin-plated cans or tin-plated steel sheets, the tin in the metallic surfaces to be treated is dissolved to a great extent, thereby reducing the gloss on the tin-plated surfaces and losing the function of the tin plating. When they are applied to steel sheets, only films in a form of smut are formed, and no coating or film with superior anticorrosive performance and adhesion of paints can be obtained. This prior art technique treats Mo as an equivalent of Sn and includes a film forming accelerator in each case. However, when the conversion coating solution containing Mo is applied to steel sheets or tin-plated steel sheets, no coating or film having good adhesion with an overcoat can be obtained.
Japanese Laid-Open Patent Application No. 62,179/1980 discloses conversion coating solutions which have a pH value in the range of 5.5 to 6.5 and contain 0.01 g/l to 0.5 g/l of stannous ions with respect to an acid alkali phosphate, and 2 to 12 parts of fluorine ions with respect to the stannous ion amount. As these solutions contain fluorine, measures should be taken to remove the fluorine from waste liquids from plants for conversion treatment from the standpoint of environmental protection. Several techniques for removing fluorine from waste liquids are known such as adsorption of the fluorine on active alumina, but the cost for treating waste liquids according to these techniques is high. As the fluorine is used to stabilize the stannous ions by forming a complex, excess fluorine is necessary. Accordingly, where tin-plated steel sheets or tin-plated DI cans are treated with such a conversion coating solution, the amount of etching of the tin surfaces is so large that the anti-corrosiveness of the tin formed on the treated metallic surfaces is weakened.